1. Field of the Invention
The present invention relates to a method for preparing an amphiphilic block copolymer in an aqueous medium and in an emulsion with no free surfactant.
2. Description of Related Art
Amphiphilic copolymers are useful in many technical fields, in particular in the paints, varnishes and coatings field. Aqueous compositions, with no organic solvent, are particularly desired, firstly owing to the fact that the water used as solvent is less expensive than organic solvents, and, secondly, owing to problems of toxicity and pollution inherent in most organic solvents.
It is known practice to prepare an amphiphilic copolymer by polymerization with the aid of a reversible addition-fragmentation chain transfer (RAFT) agent, in particular in solution in an organic solvent. However, the use of organic solvents no longer meets current non-toxicity and non-pollution criteria.
The preparation, in water, of a (co)polymer using a RAFT agent can be carried out by emulsion polymerization, but it is then necessary to add a surfactant. However, since the (co)polymer obtained is generally introduced into the various paint, varnish or coating compositions directly in the form in which it is obtained, without purification, said compositions contain the surfactant free, which can then migrate when the compositions are deposited on a substrate.
Polymerization of hydrophobic monomers in the presence of a RAFT agent can be carried out without solvent (“bulk” polymerization), but the reaction medium is then very viscous, to the detriment of the speed of polymerization and of the homogeneity of the composition of the (co)polymers obtained.
A method for preparing an amphiphilic copolymer, in water, in the presence of a macro-RAFT agent is described by Ferguson et al., [Macromolecules, 2005, 28(6), 2191-2204]. The macro-RAFT agent is obtained by reacting a RAFT agent in solution with a hydrophilic monomer (acrylic acid), in the presence of a free-radical polymerization initiator and NaOH, in an oxygen-free atmosphere, with stirring at 60° C. for 2 hours. The RAFT agent is a 2-{[(alkylsulfanyl)carbonothioyl]sulfanyl}propanoic acid, in which the alkyl group is a butyl or dodecyl group. When the alkyl group is a butyl, the reaction can be carried out in water. When the alkyl group is a dodecyl, the solubility of the RAFT agent is too low and the reaction must be carried out in an organic solvent. The block copolymer is obtained by means of a method consisting in preparing an aqueous solution containing the macro-RAFT agent, a free-radical polymerization initiator and NaOH, at 60° C., and then adding, in successive fractions, the hydrophobic monomer (n-butyl acrylate, ethyl acrylate or styrene) intended to form the hydrophobic block. The monomer is added over more than 5 hours, so as to keep the monomer concentration in the reaction medium at a value below the saturation concentration. The addition of the hydrophobic monomer in fractions complicates the method and extends the duration of the process.
A method for polymerizing monomers in water in the presence of a macro-RAFT agent is described by A. Martins dos Santos et al. [Macromol. Rapid Comm., 2007, 28, 1325-1332]. The macro-RAFT agent is of the “dithiobenzoate” type, and it is obtained by reacting a poly(ethylene oxide) having an amino end group with succinimido-4-[(phenyl-1-thioxo)thio]-4-cyanopentanoate. An amphiphilic block copolymer is prepared by means of a method during which the macro-RAFT agent, the hydrophobic monomer and a free-radical polymerization initiator are introduced into water, the reaction medium is purged so as to remove the oxygen, and then heated at 70° C. This method does not make it possible to control chain growth: it is not possible to vary the molar mass of the polymer formed according to the monomer conversion; there is no direct relationship between the number-average molar mass and the ratio of the initial concentrations of monomer and of macro-RAFT agent; the molar mass distribution is broad and analogous to that obtained by means of a conventional emulsion polymerization method.
M. Manguian et al., [Macromol. Rapid Commun. 2006, 27, 399-404] describe the preparation of a macro-RAFT agent by reacting 4-cyanopentanoic acid 4-dithiobenzoate (CPADB) with diethylaminoethyl methacrylate (DEAEMA), and the use thereof for preparing an amphiphilic block copolymer with styrene, in water, in the presence of a free-radical polymerization initiator, without the addition of surfactant. In this method of implementation also, the chain growth cannot be controlled.